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=S-Adenosylmethionine decarboxylase= S-Adenosylmethionine decarboxylase (AdoMetDC) is a key enzyme in the polyamine biosynthetic pathway, forming the amine decarboxylated S-adenosylmethionine It also aids in the synthesis of spermine and spermidine. Spermine and spermidine are polyamines that are essential growth factors and critical in cell differentiation. Their levels within cells are regulated by the amount of AdoMetDC available. Thus, AdoMetDC is tightly regulated in mammalian cells.

Structure and Function
S-Adenosylmethionine decarboxylase is a (αβ)2 dimer, forming a four-layer αββα sandwich ,. The αβ monomers both have the same structure. The β chain consists of the residues 1-67 while the α chain contains the residues 68-329. Each β sheet contains eight anti-parallel β strands. AdoMetDC has a very unique fold compared to other large β-sandwich structures as well as other pyruvoyl-dependent amino acid decarboxylases. The two β sheets are connected by only one covalent bond which allows them a large amount of flexibility to behave as independently folded domains that move with respect to each other. The α and β subunits are formed by an internal cleavage reaction.

AdoMetDC belongs to a small class of decarboxylating enzymes that use as a prosthetic group a covalently bound pyruvate. The same cleavage reaction that forms the α and β subunits also converts a serine (Ser68) residue into the pyruvate. This self processing reaction occurs via a N to O acyl rearrangement. The pyruvoyl group is bound to the N-terminal of an α subunit. Decarboxylation of S-adenosylmethionine (AdoMet) to S-adenosyl-5’-(3-methylthiopropylamine) (dcAdoMet) is catalyzed using AdoMetDC. Spermidine is the receptor of the aminopropyl group from dcAdoMet forming spermine or spermidine. This is an early step in the pathway of polyamine biosynthesis of dcAdoMet, which commits it completely to this fate.

Mechanism:
Binding of AdoMet to its enzyme AdoMetDC is the first step and binding occurs through the pyruvate prosthetic group, reacting to give a Schiff base. The pyruvate then acts as an election sink, helping to break the carbon to carboxylic acid bond (C-COO-) resulting in a carbon dioxide (CO2) being eliminated. Protonation occurs at the R carbon of the product resulting in the release of dcAdoMet. This protonation also regenerates the pyruvate cofactor so that it is available and ready for another reaction.